Geochemistry of a pristine fynbos ecosystem in the Harold Porter National Botanical Gardens and Kogelberg Biosphere Reserve

By A K Smit

Abstract

Bibliography: leaves 79-83.Water, soil and rock samples were taken from the Harold Porter National Botanical Gardens and the Kogelberg Biosphere Reserve in order to investigate interactions between the various nutrient pools in a pristine fynbos ecosystem. Physical variables (pH, EC, temperature, DO, alkalinity) were measured in the field, and water samples were collected and analysed for chemical composition. Chemical composition and mineralogy of soil and rock samples from the Peninsula, Cedarberg and Goudini formations were determined to assess the influence of parent geology on soil chemistry. The surface water chemistry in the study area was remarkably homogeneous, with only major ions Na, CI, Mg, Ca, Si and S04 exceeding concentrations of 1 ppm at any of the sample sites. Concentrations of other major ions AI, K, NH4, N03 and Fe ranged between 0.1 and 0.7 ppm. The minor and trace elements B, Sb, Sr, Zn and Mn were present in concentrations greater than 1 ppb. Compared to the seawater chloride ratio, Na, Mg, K and S04 were depleted in the streams relative to CI, showing that these elements are preferentially accumulated and are conserved within the fynbos ecosystem. In contrast, CalCI ratio is elevated in streams, indicating an external source of Ca other than the seawater. This source may be windblown dust. The Fe content of soils appears to be lower than that of the parent bedrock, especially in the case of the iron-rich Goudini Formation. This indicates an as yet undetermined pathway for iron to leave the system, which may be through deeper anoxic groundwaters in the area. Only one site from the Oudebosch River headwaters showed significant deviation from other sites in several of the elemental trends, and also had no measurable DOC content, compared to the other sites which ranged from 10 to 16 mg/L DOC. These differences are probably the result of the different vegetation (afromontane forest as opposed to fynbos) and geological setting (clay rich, Cedarberg Formation soils). Soils contained higher concentrations of water-soluble ions than the streams, and organic carbon content ranges from 6 to 34 wt%. Organic 0 horizons and Melanic/Humic A horizons constituted the major soil types. Cation exchange capacities of the soil samples ranges between 31.3-93 mmolJkg, and about half of the soil samples were determined to be acid saturated and fairly leached of ions, probably due to the high rainfall. The soil composition and mineralogy correlate well with that of the underlying bedrock, and was dominated by quartz, with some clays, micas, feldspars and hematite. The study was similar to one in the Cedarberg, which has the same geological setting and equal fynbos species richness. This is the first integrated geochemical study to characterise the pristine fynbos biome of the Harold Porter National Botanical Gardens and the Leopards Gorge River Catchment, and will allow rapid assessment of any future changes